1. Field of the Invention
The present invention relates to a belt driving mechanism, and in particular, to a belt driving mechanism for a flat belt conveyor system incorporated into an apparatus such as an electro-photographic apparatus, a printer, a printing machine, a bill inspection machine, or a mail sorter.
2. Description of the Related Art
The belt conveyor system is known as a technique for conveying various objects such as sheets, for example, print paper, bills, and tickets using a sheet-like belt having no concavities or convexities. In the field of copiers, a printing method is adopted which transfers a toner image drawn on a transfer belt, from the transfer belt to paper. In this field, the flat belt conveyor system is also used as a mechanism that conveys the transfer belt. The flat belt conveyor system is composed of a plurality of rollers including driving rollers driven by a motor or the like and a flat belt wrapped around the rollers. Rotating at least one of the driving rollers allows the flat belt to be conveyed to drive the entire mechanism.
A problem with the flat belt conveyor system is that the belt may be displaced laterally with respect to a belt driving direction. For example, in the formation of a color image, the lateral displacement can not register various colors when images of the colors are superimposed on one another. It is important for the flat belt conveyor system to accurately control the position of the belt.
A factor causing the lateral displacement of the belt is the tilt of the rollers. When the belt is conveyed with the axes of a driving roller and a driven roller not parallel to each other, the rotating direction of the rollers may be tilted from an intended belt advancing direction. A lateral force thus acts on the belt, which is consequently displaced laterally. Even an inclination equivalent to a design error may laterally shift the belt. A mechanism is thus required which avoids displacing the belt or corrects the belt position.
The following methods have been proposed to prevent the belt from being laterally displaced: providing ribs such that the opposite edges of the belt are caught on the rollers, forming the opposite ends of the rollers into flanges, or shaping the belt like the letter T or the like, that is, so that it has concaves and convexes and driving the belt using rollers having concaves and convexes that are fitted together.
The flat belt can also be run stably by using a crown roller the middle of which is slightly swollen like a drum. To prevent the flat belt from being laterally displaced without using the crown-face roller, a tension roller or the like may be placed and forcibly tilted in response to the lateral displacement of the belt.
Another method for preventing lateral displacement is to fix the running position of the belt using a pole or a guide. This method causes the belt to be always rubbed during driving, thus disadvantageously shortening the life of the belt or apparatus.
Jpn. Pat. Appln. KOKAI Publication No. 7-157129 proposes a method of correcting the belt position using a brake. This proposal uses the brake to exert a braking force on a part of the belt to vary the tension on the belt surface, thus suppressing the lateral displacement.
The above belt driving mechanism is incorporated into an image forming apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-45700, a bill inspection machine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-96896, a mail sorter disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-338854, and the like. The belt driving mechanism is an important component of these apparatuses.
As described above, the belt can be prevented from being laterally displaced by the method of providing ribs such that the opposite edges of the belt are caught on the rollers, forming the opposite ends of the rollers into flanges, or shaping the belt so that it has concaves and convexes, for example, like the letter T and driving the belt using rollers having concaves and convexes that are fitted together. However, these methods forcibly suppress the force displacing the belt laterally to exert an unnatural force on the belt to disadvantageously shorten the life of the belt. Further, if the concaves and convexes of the belt increase its thickness, a stronger force is required to bend the belt. A heavy burden is thus imposed on the conveyor system to also disadvantageously shorten the life of the apparatus. Thus, it is desirable to use a flat belt with a minimum number of concaves and convexes.
With the method of using the crown-face roller, the belt is slightly bent along the middle of the roller during conveyance. Thus, disadvantageously, the crown roller cannot be used in, for example, an apparatus such as a intermediate transfer belt in an image forming apparatus in which bending of the belt may distort the image.
The method of forcibly tilting the tension roller or the like increases the number of parts required and thus the size of the conveyor system. This is disadvantageous in terms of cost and space.
The method proposed by Jpn. Pat. Appln. KOKAI Publication No. 7-157129 uses the brake to exert a braking force on a part of the belt to vary the tension on the belt surface. The lifetime of the belt is shortened because the belt is always rubbed during driving by the method.
The above problematic belt driving mechanism is applied to the image forming apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-45700, the bill inspection machine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-96896, the mail sorter disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-338854, and the like. However, the belt driving mechanism has posed various problems in the above apparatuses; processing accuracy may lower as a result of the lateral displacement of the belt and the lifetime of the apparatus may be shortened.